1. Field of the Invention
This invention relates to an improvement applicable to a wire electrode type electrical discharge machining apparatus. More specifically, this invention relates to an improvement applicable to a taper-machining controller for the purpose of enabling a wire electrode type electrical discharge machining apparatus to perform taper-machining with high accuracy, even in situations in which the hardness or rigidity of a wire electrode varies due to variations of the temperature thereof following variation of the magnitude of electric current flowing therein.
2. Description of the Related Art
Wire electrode type electrical discharge machining is classified into two independent categories. The first category ordinary machining, wherein a wire electrode is maintained in a direction perpendicular to the direction in which a workpiece is moved during a machining process. The second category is taper-machining, wherein a taper-angle, which is defined as an angle between the direction of a wire electrode and the direction perpendicular to the direction in which a workpiece is moved, or as an angle by which the wire electrode is inclined, is maintained at a value which is not zero degrees, or is varied, during a machining process.
Referring to FIG. 1, taper-machining is realized by controlling a taper-angle .theta. which is determined by the direction 22 which is perpendicular to the lower plane 21 of a workpiece 2 and the direction of a wire electrode 1 (an angle by which a wire electrode is inclined (from vertical). In the case where the wire electrode 1 is sharply bent at the vertexes A, B, as shown in FIG. 1, taper-machining can be realized by using a servomechanism which moves one of the vertexes A, B in a plane parallel to the lower plane of the work 2 or perpendicular to the wire electrode 1 (or from side to side in the drawing).
However a wire electrode 1 is elastic, and the magnitude of tension required to be applied to the wire electrode 1 during an electrical discharge machining process should be determined based on the diameter of the wire electrode 1. Therefore, referring to FIG. 2, particularly in instances where die type guides are employed for guiding the wire electrode 1, vertexes AA, BB, which are determined on the basis that the wire electrode 1 is sharply bent, do not conform to points C, D, at which the wire electrode 1 is actually detached from each of the upper and lower wire electrode guides 3, 4. Further, since the magnitude of this nonconformity between the vertexes AA, BB and the points C, D is not necessarily constant but varies depending on various conditions, a less than satisfactory magnitude of accuracy is caused in wire electrode type electrical discharge machining.
This drawback can be removed by employing a servomechanism which moves one of the assumed hypothetical vertexes, paying attention to various parameters, including the elasticity of the wire electrode 1, the required magnitude of tension to be applied to the wire electrode 1 during an electrical discharge machining process, the potential magnitude of nonconformity between the aforementioned vertexes and the actual points at which the wire electrode 1 is detached from each of the upper and lower die type wire electrode guides 3, 4, and the like. Particularly because a wire electrode type electrical discharge machining apparatus necessarily employs a computer, such as a numerical control system, a wire electrode type electrical discharge machining apparatus can readily be provided with this type of improved servomechanism, which corrects the points along which a servomechanism moves for taper-machining. Therefore, I completed an invention directed to a wire electrode type electrical discharge machining apparatus providing such a servomechanism as is described above, wherein a computer is employed to apply the aforementioned correction to the point which is moved by a servomechanism for taper-machining. An international patent application, No. PCT/JP86/54 was filed in the Japanese Patent Office as the receiving office on Feb. 7, 1986 for this invention.
However, this newly developed wire electrode type electrical discharge machining apparatus still has a further drawback to be removed. The above-mentioned apparatus lacks a means for correction of potential harmful influences caused by variation of hardness or rigidity of the wire electrode due to variation of the temperature therein due to variation of the magnitude of electric current flowing therein. The above-mentioned apparatus is provided with a servomechanism which controls the taper-angle .theta. by moving hypothetical vertexes, which are assumed, paying attention to various parameters, including the elasticity of the wire electrode, the required magnitude of tension to be applied to the wire electrode during an electrical discharge machining process, the potential magnitude of nonconformity between the aforementioned assumed vertexes and the actual point at which the wire electrode is actually detached from each of the upper and lower die type wire electrode guides, and the like. Therefore, this improved type wire electrode type electrical discharge machining apparatus is provided with the aforementioned taper-machining controller, and address the drawback wherein the machining error slightly increases whenever the magnitude of the machining current varies.